1. Field of the Invention
This invention relates generally to the field of spectrometry, and more particularly, to a method and apparatus for dispersive holographic spectrometry.
2. Description of the Related Art
When atomic particles are excited by electromagnetic radiation of certain frequencies, they may absorb some of the radiation at specific wavelengths and give off the energy in other forms, such as electromagnetic radiation of different wavelengths or rotational or vibrational energy. By irradiating a sample with infrared radiation and detecting the transmitted intensity of the infrared radiation over a certain range of frequencies, a spectrum of the wavelengths absorbed by the sample can be generated over that range of wavelengths of the infrared spectrum. Since different atoms will absorb different wavelengths of radiation, the infrared absorption spectrum of each atom or molecule is unique. By knowing at what wavelength certain molecules will absorb infrared radiation, the elemental and molecular constituents of a sample can be determined by comparing the absorption spectrum of a sample to the absorption spectrum of a reference sample at the same intensity of radiation.
Spectral information can be measured by two methods generally referred to as the dispersive method and the interferometric method. For the interferometric method, the electromagnetic radiation is divided into at least two paths and then recombined in an interference pattern. The interference pattern is measured to give the spectral information. The dispersive method separates the radiation into component wavelengths by means of a grating or a prism. Each set of component wavelengths is then individually measured.
One form of interferometric holographic spectrometry is disclosed in U.S. Pat. No. 4,779,984 to Cook. In that patent, an infrared source emits a beam of radiation towards a reimaging mirror which, in turn, focuses the infrared radiation on a relatively small aperture accessing a holographic spectrometer. The holographic spectrometer divides the incident radiation into two beams by means of separate light guides. Each of the light guides then directs the radiation through a single geodesic lens to collimate the beams. The two light beams are combined at an array of detectors in an interference pattern. The interference pattern can then be interpreted to give spectral information.
The above-referenced spectrometer system suffers the drawbacks of requiring a reflector to focus the radiation into a relatively small aperture on the holographic spectrometer itself. This requires high precision adjustment using servomechanisms to direct the radiation from the reflector into the aperture. Such mechanisms are expensive and require a significant amount of time for alignment.